1. Field of the Invention
This invention relates to support circuitry for operating high speed modems, and particularly to a compensated hybrid wiring configuration in which echo attributable to internal transformer resistance is minimized.
2. Description of the Related Art
High speed modems typically require high performance support circuitry which may include transmission and receiver circuitry, combined in a transceiver, and data access arrangement circuitry. A hybrid coupling circuit, typically called a hybrid, couples the modem transceiver to an external telephone line. In such hybrid configurations, two wires extending from the transceiver are designated for output transmission to the telephone line, while an input line leads from the telephone line to the modem receiver.
The hybrid typically includes a transformer coupled between the external telephone line and the transceiver transmit and receive lines. The transformer, however, is associated with several inherent drawbacks, namely, distortion and echo characteristics which could significantly impair the overall modem transmission and reception performance. To minimize distortion, the transformer will be physically large. Accordingly, to maintain the proper transformer ratio, multiple windings around each transformer leg are necessary. The windings, however, tend to produce high internal DC resistance (DCR) within the transformer.
While distortion is a characteristic of the transformer alone, unwanted echo Is attributable to a combination of factors including the transformer internal DCR and mismatched impedances between the hybrid circuitry and the external telephone line. Echo, to a certain extent, is a desirable feature of telephone systems. During speech, echo allows the speaker to hear "side tones" of his or her own speaking voice over the telephone receiver. The desirable echo level, however, must be controlled so that the telephone users can hear themselves, as well as each other.
Modems, in contrast, do not require echo. A modem generally does not have to hear itself since a known, discrete amount of data is transmitted or received by the modem without necessitating any intermediate responses. Thus, the modem does not require reflection tones to indicate the speech level, etc. However, due to the internal DCR of the transformer which generally produces mismatched impedances between the transceiver and the telephone line, undesirable reflected echo may be generated. Thus, although it has been found that to minimize distortion, the transformer DCR must be increased, echo is also undesirably increased due to the higher DCR.
A characteristic of hybrid arrangements is trans-hybrid loss (THL). It has been found that to minimize undesirable echo on the modem lines, the THL must be maximized. As described above, however, the transformer DCR and distortion are inversely related. If the distortion is low, the DCR is high. Yet, a high DCR value results in high echo levels since the THL is dependent on the DCR. The mismatch of impedance within the hybrid causes higher echo which lowers the THL.
Accordingly, different hybrid circuits have been developed to meet the various requirements of modem support circuitry. These requirements include the following: (1) to provide proper internal impedance values to compensate for the transformer internal DCR such that the internal hybrid impedance matches that of the external telephone line; (2) to present a known, standard fixed loss (in dB) to the modem between the hybrid and the telephone line; (3) to provide sufficient feedback voltage to cancel the reflected echo. In this way, the incoming and outgoing signals can be distinctively received and transmitted, while the deleterious effects of transformer distortion and echo on modem performance are minimized.
For example, FIG. 1 illustrates a two wire telephone line interface 100 coupled to a conventional hybrid 110 and transceiver 120 arrangement. The transceiver functions as both a receiver and transmitter, and generally includes two transmit lines Tx1 and Tx2, and one receive-input line Rxin. Outgoing signals are sent from the transmit lines, while an incoming signal from the external telephone line is received on the Rxin line. Node 130 represents the hybrid point at which the incoming and outgoing signals may be superimposed. A transformer 140 is typically coupled between a matching resistor Rm and the telephone line 100.
As described above, the THL is dependent upon the transformer internal DCR which, in turn, tends to be proportional to the echo. To decrease echo, the THL must be increased. The THL is presented at the hybrid point 130, where the transmitted signal from Tx1 meets the received signal input into Rxin, in addition to the transmitted signal through the transformer. Due to the transformer DCR and the matched impedance requirement, however, the voltage levels imposed on the hybrid point 130 is imbalanced, producing an excess voltage which results in the undesirable echo directed into the receive input line.
To minimize power loss and echo, the internal impedance of the hybrid 110 should closely match the impedance of the telephone line 100. That is, the total impedance of the transformer and the matching resistor Rm Is preferably equal to the telephone line impedance. Thus, if the telephone line 100 has a 600 ohm impedance, and the transformer DCR is 100 ohms per winding, for a total resistance of 200 ohms, then to match the line, a 400 ohm matching resistor Rm is incorporated in the hybrid so that 600 ohms are seen on both sides of the telephone line 100 and the hybrid 110.
More simply, if an ideal transformer having zero resistance is used, and if the telephone line resistance is 600 ohms, the matching resistor value must be 600 ohms. If a voltage Vtx1 is output on the Tx1 line, and a voltage Vtx2 is output on the Tx2 line, then the voltage at the hybrid point 130 is zero since the Tx2 signal is merely an inverted form of the Tx1 line. Thus, the voltages at the hybrid point are canceled such that no echo is heard at the receiver, but only the telephone line input signal. However, because such ideal transformers are not typically used, most transformers have some resistance which may significantly affect the voltage level at the hybrid point, thus causing echo on the receiver line.
For example, returning to the example above in which the transformer resistance totals 200 ohms, requiring a 400 ohm matching resistor, the voltage at the hybrid point is one-third the voltage on the transmit lines. Consequently, the receiver will receive an echo having a voltage level of Vrx=1/3*Vtx1 since the drop of the outgoing transmitted signal through Rm is smaller than that through the 800 ohm total impedance (on the right side of the hybrid point 130 in FIG. 1). Thus, the receive input line receives both the received input signal as well as the echo comprising one-third of the voltage at Tx1. As a result, since the incoming received signal is expected to be 10 to 30 dB lower than Vtx, the receiver may not be able to distinguish between the desired signal and the echo.
Other hybrid designs have also been developed to reduce the effects of echo on modem performance. As shown in the hybrid of FIG. 2, an operational amplifier 200 ("op amp") having a differential input is connected to the Rxin line. The op amp 200 has several input connectors including: (1) a complex signal at pin 2 received via a feedback resistor network coupled to the transformer 210 and telephone line 220, including any reflected echo from the transformer, and (2) the voltage output via the resistor network 230 from the transmit line Tx1. Although the reflected transmission echo signal is still generated and sent back on the receive line, it is input into the inverting input pin of the op amp 200, illustrated in FIG. 2, while the resistor network 240 coupled to the Tx1 line forces the transmit voltage into the noninverting input of the op amp 200. Upon adding the two input signals, the output from the op amp 200 comprises generally the desired incoming signal since the voltage signals input into the inverting and noninverting inputs of the op amp 200 cancel each other.
In the above-described op amp configuration of FIG. 2, however, the values of the resistors comprising the resistor networks 230 and 240 must be accurately varied and adjusted to produce the desired voltage canceling effect to thereby minimize the echo. This system is generally accurate. However, such an op amp arrangement tends to add undesirable non-linear distortion, an inherent characteristic of operational amplifiers. In addition, such an op amp hybrid requires a substantially increased number of components, which significantly increases the complexity and cost of the overall circuit. Thus, the additional wiring, resistors, capacitors, and the op amp disadvantageously increase the complexity and size of the circuit, as well as the cost. The added external components also generate noise and distortion, thereby further limiting modem performance.